Class-D amplifiers are used in audio amplification circuits for a wide variety of products such as MP3 players, cellular telephones, and stereo audio amplifiers. Their use has become pervasive, in part, because of their high efficiency and their ability to easily interface with digital audio circuits. A typical class-D output stage drives an output load by switching the load between different power supplies at a frequency greater than the bandwidth of the desired output signal. The switching energy is electrically and/or acoustically filtered by the characteristics of the particular load circuit. For example, if a class-D amplifier drives loudspeaker at a switching frequency higher than the audible frequency band, audio frequencies are converted to sound energy and the high frequency switching energy is filtered by the mass and/or inductance of the loudspeaker. Higher efficiencies are achieved when switching losses are minimized across the output switches of the class-D output stage. Because a class-D amplifier can receive and drive a pulse modulated signal, a class-D output stage can be interfaced directly to a digital output circuit without the need for precision analog components in the direct audio signal path.
Furthermore, a high dynamic range, which results in higher audio fidelity, can be achieved by using oversampled digital-to-analog conversion techniques, such as sigma-delta modulation, to provide a pulse modulated input signal to a class-D output stage. In most cases, however, a high-order sigma-delta modulator is limited to only a portion of the total possible output dynamic range because of instabilities that occur when the in-band output signal approaches 100% of the maximum modulation level. Many systems, therefore, limit the output of a sigma-delta DAC to about 50% of it maximum output using a maximum modulation range of 50%. For example, if a single ended operating class-D amplifier operates on a 10 volt power supply, the maximum in-band output signal is equivalent to only about 5V peak to peak when the maximum modulation range is set to 50%.